The problem of how afferent synaptic connections and activity influence nervous system development is a juncture at which the biology and psychology of organismic development meet. The early experience of animals can have profound effects on their subsequent behavior and on the structure and physiology of their nervous system. Yet, at the cellular level, "experience" can best be thought of as a change in the afferent connections or activity impinging on neurons. Thus, the study of how afferent connections influence neuronal development is central to further understanding of both neuroembryology and of the manner in which early sensory experience influences brain and behavior development. The proposed experiments will examine with light and electron microscopic methods the morphological development of two avian brain stem auditory nuclei n. magnocellularis and n. laminaris. These experiments will contrast the normal ontogeny of these neurons with their development after embryonic deafferentation or early restriction of the acoustic environment. These experiments will bring together manipulations of morphological and experiential variables in order to examine a wide range of afferent influences in the early life of two functionally-related neuronal types with many advantages for such studies. A second objective of the proposed studies is to provide neuropathologic information on the central neural effects accompanying early acoustic deprivation or receptor damage. Several of the proposed experiments involve early manipulations of the middle and inner ears or of early acoustic experience, with subsequent study of morphological development in second-order and third-order auditory in the brain. It is hoped that these studies will provide a detailed and comprehensive survey of the effects on auditory neuron development of early conductive and sensorineural hearing loss.